The present invention relates to ablation instrument systems that use electromagnetic energy in microwave frequencies to ablate internal bodily tissues. More particularly, the present invention relates to an antenna arrangement in an ablation instrument system that directs the microwave energy in a selected direction.
Catheter ablation has recently become an important therapy for certain cardiac arrhythmias, cardiac disrhythmias and tachycardia. Many ablation instrument systems utilize radio frequency (RF) energy as the ablating energy source. accordingly, a variety of RF based catheters and power supplies are currently available to electrophysiologists. However, radio frequency energy has several limitations including the rapid dissipation of energy in surface tissues resulting in shallow xe2x80x9cburnsxe2x80x9d and failure to access deeper arrhythmic tissues. Another limitation of RF ablation instruments is the risk of clot formation on the energy emitting electrodes. Such clots have an associated danger of causing potentially lethal strokes in the event that a clot is dislodged from the ablation instrument.
A second common ablation approach is the use of high voltage, direct current defibrillator discharges. Direct current ablation has several drawbacks including the need for general anesthesia and explosive discharges that can cause debris or even rupture certain cardiac organs. For these and other reasons, significant attention has been given recently to alternative ablative energy sources.
Microwave frequency energy has long been recognized as an effective energy source for heating biological tissues and has seen use in such hyperthermia applications as cancer treatment and preheating of blood prior to infusions. In view of the drawbacks of the traditional catheter ablation techniques, there has recently been a great deal of interest in using microwave energy as an ablation energy source. The advantage of microwave energy is that it is much easier to control and safer to apply than direct current applications. Moreover, microwave energy is capable of generating substantially larger lesions than RF catheters, which greatly simplifies the actual ablation procedures. Accordingly, there are a number of catheters under development which utilize electro-magnetic energy in the microwave frequency range as the ablation energy source. By way of example, such systems are described in the U.S. Pat. No. 4,641,649 to Walinsky; U.S. Pat. No. 5,246,438 to Langberg: U.S. Pat. No. 5,405,346 to Grundy, et al.; and U.S. Pat. No. 5,314,466 to Stern, et al, each of which is incorporated herein by reference.
While these microwave ablation instruments may be employed to adequately ablate bio-tissue, most of the existing microwave ablation instruments contemplate the use of longitudinally extending helical antenna coils that direct the electromagnetic energy in a direction that is generally perpendicular to the longitudinal axis of the instrument although the fields created are not well constrained to the antenna itself. Although such instrument designs work well for a number of applications, it would also be desirable to provide microwave ablation instrument designs that are capable of effectively transmitting electromagnetic energy in other specific directions, such as a generally forward direction relative to the longitudinal axis of the instrument.
One such end-firing ablation instrument is disclosed in our U.S. Pat. No. 5,800,494. This instrument design incorporates a coaxial transmission line having an inner conductor, an outer conductor and a dielectric material medium disposed therebetween. At the distal end of the inner conductor is a spirally wound antenna coil adapted and oriented to emit an electromagnetic field in a direction distal to the longitudinal axis at the tip of the inner conductor.
Although this design provides functional distal firing capabilities, the width of the electromagnetic field is only about 2-4 mm wide. Accordingly, without substantially increasing the antenna dimensions and/or power output, use of the ablation instrument is limited to relatively small target regions.
The present invention provides a microwave ablation instrument including a transmission line having a first conductor and a second conductor suitable for the transmission of microwave energy, and having a proximal end thereof coupled to a microwave energy source. The ablation instrument further includes a horn antenna device having an outer conductor and an inner conductor. The outer conductor of the horn antenna is electrically coupled to the second conductor of the transmission line, and includes a peripherally extending interior wall flaring outwardly to define a recess therein. The inner conductor of the horn antenna is electrically coupled to the first conductor of the transmission line and terminates in the outer conductor recess. The inner conductor and the outer conductor cooperate to emit an electromagnetic field sufficiently strong to cause tissue ablation in a direction generally away from the flared interior wall of the outer conductor.
In one embodiment, the outer conductor of the antenna device is integrally formed with the outer second conductor of the transmission line. The inner conductor of the antenna device is further integrally formed the inner first conductor of the transmission line.
In another aspect, the line dielectric material includes a first dielectric constant, and the antenna dielectric material includes a second dielectric constant. A proximal face of the antenna dielectric material is in conductive contact with the line dielectric material, and a distal face of the antenna dielectric material is spaced-apart from the proximal face by a predetermined distance enabling resonance of the emitted electromagnetic field at the distal face proportionately based upon the first dielectric constant and second dielectric constant.
In another embodiment of the present invention, a horn antenna assembly is provided for use with a microwave ablation instrument including a coaxial transmission line having a line inner conductor and a line outer conductor separated by a line dielectric material. The horn antenna assembly includes an antenna outer conductor electrically coupled to the line outer conductor. The antenna outer conductor includes a peripherally extending interior wall defining a recess and which flares outwardly from a proximal end to a distal end thereof. An antenna inner conductor is electrically coupled to the line inner conductor and adapted to terminate in the recess of the antenna outer conductor. The antenna inner conductor and the antenna outer conductor cooperate to emit an electromagnetic field sufficiently strong to cause tissue ablation. This field is radiated in a direction generally away from the distal end of the horn antenna assembly. The present inventive horn assembly further includes an antenna dielectric material having a second dielectric constant and being disposed in the recess of the antenna outer conductor. The antenna dielectric material includes a proximal face in conductive contact with the line dielectric material and a distal face spaced-apart from the proximal face by a predetermined distance enabling resonance of the emitted electromagnetic field at the distal face. This resonance is proportionately based upon a first dielectric constant of the line dielectric material and the second dielectric constant of the antenna dielectric material.
In one configuration, an impedance matching device is included having a third dielectric constant. A proximal face of the impedance matching device is in conductive contact with the distal face of the antenna dielectric material, and a distal face of the impedance matching device is formed for direct contact with a targeted tissue to be ablated. The distal face of the impedance matching device is spaced-apart from the proximal face thereof by a second predetermined distance. The third dielectric constant and the second predetermined distance cooperate with electromagnetic field discharged from the antenna dielectric material distal face such that the impedance of the emitted electromagnetic field at the distal end of the impedance matching device is substantially matched with the impedance of the tissue to be ablated.
In still another embodiment, a microwave ablation instrument includes a coaxial transmission line including a line inner conductor and a line outer conductor separated by a line dielectric material. The transmission line, having a first dielectric constant, defines a transmission face thereof, and a proximal end thereof coupled to a microwave energy source. An antenna device is coupled to the transmission line at the transmission face to emit an electromagnetic field sufficiently strong to cause tissue ablation. The antenna device further includes an antenna dielectric material having a proximal face in conductive contact with the line transmission face, and a distal face spaced-apart from the line proximal face. An interference device is positioned in the first dielectric material between the line outer conductor and the line inner conductor, and further at a location behind the line transmission face. The interference device is spaced-apart from the distal face of the dielectric material a predetermined distance such that resonance is created in the horn antenna.
In another embodiment, the interference device is provided by an annular ridge having a reflection face extending from the line outer conductor towards the line inner conductor. The reflection face is preferably substantially planar and substantially parallel to the distal face of the antenna dielectric material. In another arrangement, the annular ridge includes a transition wall portion extending rearwardly from the reflection face, and smoothly tapering from the reflection face to the outer conductor.